Amplifier cascade with negative feedback



June 13, 1939. J. BABLER AMPLIFIER CASCADE WITH NEGATIVE FEEDBACK Filed April 2, 1936 M m 5 5 5 A w Patented June 13, 1939 am'urmn CASCADE WITH NEGATIVE FEEDBACK I Justus Babler, Zurlch-Albisrieden, Switzerland,

assignor to Siemens & Halske, Aktiengeselischaft, Siemensstadt, near Berlin, Germany, a corporation of Germany Application April 2, 1936, Serial No. 72,328 In Germany April 15, 1935 e 6 Claims.

As is known, the distortions appearing in an amplifier, and more especially the non-linear distortions, can be reduced when employing a negative feed-back. The reduction of the distortion is thereby the more favorable, the higher the degree of the negative feed-back can be chosen. However, in the practical embodiment of circuits with negative feed-back considerable difllculties are encountered owing to phase rotations withinlo the feed-back circuit. If the phase conditions are so chosen that within a certain frequency range, the feed-back is produced in the correct sense, i. e., a negative feed-back, it is unavoidable in the hitherto employed circuits, that for other frequencies, the coupling assumes the form of an instantaneous coupling (positive feed-back).

While, in employing the negative feed-back in a single tube, it is comparatively easy to retain the desired phase relations within a wide frequency range, hitherto it has in practice been impossible to avoid self excitation in the case of cascade circuits in which the negative feed-back is extended over several tubes. The undesirable phase rotations must be traced in the first place to the inductances and capacities used between the individual tubes for the purpose of coupling. While it is possible to reduce the detrimental phase rotation by a suitabledime'nsioning of these coupling elements, it is not possible however, to eliminate this phase rotation entirely. a In accordance with the present invention, these difiiculties are overcome by coupling the tubes, between which the negative feed-back is provided, galvanically to each other. While this mode of coupling is known as such, .it furnishes in circuits with negative feed-back, the great advantage that undesirable phase rotations can be avoided. In this way it has become possible for the first time to .utilize a negative feed back of 40 considerable value.

For practicing the invention, it is principally immaterial at which place the resistor serving for the coupling between both tubes is inserted in the plate circuit. However, it will be advisable 5 to place the resistor between cathode and minus pole of the plate voltage source, since in this case the phase conditions will be especially favorable. This circuit affords at the same time the further advantage that a part of the voltage drop oc- 50 curring through the coupling resistor, can be utilized as grid biasing potential for the tube, in whose plate circuit the coupling resistor is inserted. If the negative feed-back is employed in an amplifier cascade, containing stages in 55 push-pull, a minimum of distortion can be attained, since the push-pull connection as such has already a small noise factor. Where a power output of high value is desired, it will be advisable to use as end stage of the cascade a class B pushpull amplifier. The preliminary stages may 5 hereby be operated either in ordinary connection or likewise as push-pull stages.

Various means are available for deriving the negatively return-coupled voltage from the plate circuit. The feed-back potential may be tapped 10 either from an ohmic series or parallel resistor placed in the plate circuit, or it may be derived from a transformer, or from a special winding of a transformer inserted in the plate circuit for coupling the consumer or load circuit. Finally, it 5 is also possible, to derive the feed-back potential at special taps of the output transformer. .When using solely ohmic means in the feed-back path, the desired phase can be obtained by selecting a suitable number of tubes over which the negative feed-back is extended.

Fig. 1 of the accompanying drawing represents by way of example an embodiment of the invention. Fig. 2 illustrates an alternative form of a portion of Fig. 1. The tubes 3, 4 and I, 2 are respectively a preliminary push-pull stage and a push-pull end stage. The potential E1 to be amplified is applied to the cascade across a pushpull transformer having the windings I, 8, 9, and the output potential E2 is derived acrossa push- 3o pull transformer having the windings III, II, l2, 13, I4, and IS. The coupling between the tubes is procured by means of resistors 5 and 6 placed between the cathodes of the preliminary tubes and the minus pole of the plate battery Bp. A grid biasing battery B; may furthermore be inserted between the common point A of the coupling resistors and the plate battery. The grids of the end tubes are galvanically coupled to the cathodes of the preceding tubes. feed-back is derived from the plate circuits of the two end tubes I and 2, across windings l0 and II which are included in the respective grid circuits of the preliminary tubes.

The grid biasing batteries l6 and II can be omitted if the leads from the feed-back windings Ill and II to the cathodes of the preliminary tubes are not directly connected to the cathodes, but are placed at corresponding tap points of the coupling resistors 5 and 6, the connection being shown in Fig. 2 in connection only with winding l0 and resistor 5 for tube 3. a As' already pointed out, instead of deriving the feed-back potentials inductively from the plate The negative 40 circuit. teed-back potential may also be tapped trom'ohmlcresistors. This circuit havetobemedespeciallywhenthesmallphase rotation produced by the inductive coupling reveals'an unallowably high value while a high degree of linearity is required, in other words, at a very high degree of reed-back.

- What I claim is: r I

1. An amplifier circuit comprising a palrof tubes connected in cascade, means for coupling the output or the second tube to the input of the first tube to produce negative feed-back, and a direct connection from the cathode of the first tube to'the control grid oi the second tube.

1'. An amplifier circuit according to claim 1, wherein a-common plate current source is provided for both tubes, and a resistor is connected between the cathode of the first tube and the negative terminal of the plate current source, said resistor serving to impress a negativebias on the control grid 0! the second tube. I

3. A vacuum tube amplifier comprising successive push-pull stages which are coupled to one another by means oi direct connections from the cathodes or the first stage to the control grids oi the second stage, characterized in that teed-back voltage between the stages is derived from two separate pairs oi coupled windings, each pair comprising a winding included in the input to a areas r tube of the firststage and a winding includedin the output oia tube or the second stage.

4. A circuit arrangement comprising a first tween grid and cathode thereof and including a winding a coupling resistor connected to the a cathode and included in the'output circuit of said first stage, a second discharge tube having its grid directly connected to the resistor, and a winding in the output circuit of said second discharge tube coupled to the winding included in the input circuit of the first discharge tube.

5.A circuit arrangement comprising a first push-pull stage, a pair of series-connectedn- *'sist0rs connected between the cathodes oi-said stage, a ucond push-pull stage having its cathodes connected to the common terminal of said resistors and its grids directly to the cathode side of said resistors, and means for coupling the second stage output circuit to he first stage input circuit to provide negative i -back betweensaid stages.

discharge tube, an input circuit connectedbe- 

